Phase change materials (PCM's) in which the heat of fusion of various hydrated salt compositions is employed are well known in the literature. In the ASHRAE Journal of September, 1974, entitled SOLAR ENERGY STORAGE, Dr. M. Telkes evaluated the thermal, physical and other pertinent properties of PCM's on the basis of economics, applicability, corrosion, toxicity and availability for large scale installations. Among the materials evaluated were various salt hydrates and their eutectics including CaCl.sub.2.6H.sub.2 O which undergoes several phase transitions to materials of different crystal structure, i.e. CaCl.sub.2.6H.sub.2 O to CaCl.sub.2.4H.sub.2 O+2H.sub.2 O at 29.degree. C.
When heated to a temperature of above 33.degree. C., the salt CaCl.sub.2.6H.sub.2 O dissolves completely in its water of crystallization. When cooled, formation of four different crystal forms is possible, i.e., CaCl.sub.2.6H.sub.2 O and three forms of CaCl.sub.2.4H.sub.2 O. If any of the 4H.sub.2 O crystals form, the heat of fusion is much less than 46 cal/gm (CaCl.sub.2.6H.sub.2 O in substantially pure form undergoes a liquid/solid phase transition at about 30.degree. C. releasing or alternately absorbing about 46 calories of heat per gram). Despite the relatively low cost of CaCl.sub.2, the formation of its four different crystal forms was deemed to be disadvantageous.
Carlsson et al., in Swedish Pat. No. 410,004, claim a method for suppressing the tetrahydrate formation during repeated melting and crystallization of a system based on CaCl.sub.2.6H.sub.2 O. In a comparative study, Carlsson et al., determined that in solutions in the concentration range of from 48 to 53 weight percent CaCl.sub.2, using CaCl.sub.2.6H.sub.2 O of highest purity, the crystallization temperatures for CaCl.sub.2.6H.sub.2 O and CaCl.sub.2.4H.sub.2 O where such that the solution was incongruently melting and that CaCl.sub.2.4H.sub.2 O crystallized and precipitated out of the solution thus losing its heat storage capacity. By using a solution of the same concentration from CaCl.sub.2 of technical grade (Road Salt) containing NaCl and KCl as impurities, the solubility of the tetrahydrate decreased and that of the hexahydrate increased and on repeated melting and crystallization, the precipitation becomes significant and the system again loses its heat storage capacity. Thus, the conclusion can be drawn that the use of technical grade CaCl.sub.2 (Road Salt) results in a poorer performance due to a relative increase in tetrahydrate formation as compared to a system based on high purity CaCl.sub.2. Carlsson et al., discovered that the addition of one or more compounds, including about 2 weight percent SrCl.sub.2.6H.sub.2 O, increased the solubility of the tetrahydrate and suppressed tetrahydrate formation on repeated melting and crystallization. The amount of addition was found to be dependent upon the amount of impurities present in the system, which in an example using Road Salt was determined to be 2.2 weight percent.
The relative amounts of each impurity in the technical grade salt (Road Salt) was not determined nor was it held to be important to the outcome of the tests conducted. In fact, the use of Road Salt was found to be less desirable from the standpoint of tetrahydrate formation compared to CaCl.sub.2 of high purity. Neither was there any recognition by Carlsson et al., that impurities of NaCl and KCl in the composition could be beneficial in reducing tetrahydrate crystal formation in such phase change compositions.
Heat storage compositions are ideally packaged in individual encapsulating means for use in conjunction with solar heating systems. Exemplary of suitable known encapsulating means for the heat storage compositions herein described are water impervious films or foils of plastic/metal laminates. Closed cell plastic foams have also been suggested in which the PCM may be encapsulated within the cells of the foam structure as illustrated in, for example, U.S. Pat. No. 4,003,426. Other useful encapsulating means are concrete, metal or plastic containers, pipes, and the like.